Insulator -band structure

In an insulator the valence band is full and the next available energy states are in a higher band called the conduction band, separated by a forbidden region. 

For an electron to migrate through the material it must gain sufficient energy to jump from the valence band across the band gap, E_g, into the conduction band. In an insulator this gap is of the order of 10 eV, much greater than can be surmounted by thermal excitation. The electrons are immobile and the material is unable to pass an electric current, however great the electric field.

See conductor and semiconductor for comparison.